Those familiar with the storage of fluids have long recognized that significant problems arise when attempting to separate a non-cryogenic liquid from a gas (vapor) in a storage vessel in a low-gravity environment. At a zero or near zero gravity, the difference in specific gravity between the liquid and the gas becomes of less consequence in determining the position of the liquid and the gas (vapor) within the vessel, i.e., a conventional gravity orienting force is not present. Accordingly, a conventional bottom liquid drain port and an upper gas/vapor vent port cannot achieve the desired separate flow of liquid and gas/vapor, respectively, from these ports.
The problem associated with zero gravity liquid/gas separation arises during space travel and, particularly, for constant velocity, non-spinning orbiting vehicles and space craft. While this problem does not arise for "blowdown" systems wherein gas is not added to the storage tank as liquid is removed, some systems preferentially add gas to the tank, e.g., to expel liquid from the tank. For example, during re-supply of a non-cryogenic liquid to a storage tank of an in-orbit space craft having a liquid in physical contact with a gas, gas must be vented from the vessel as liquid is added to prevent a pressure increase in the vessel which might otherwise rupture the light-weight vessel. If the space craft is in a constant velocity, non-spinning mode, neither acceleration nor centrifugal forces are available for liquid/gas separation. The venting of typical non-cryogenic liquids used in space travel, such as liquid fuel, with the gas must be avoided both to prevent the loss of fuel transported to the space craft at a high cost, and to minimize safety risks and space craft equipment deterioration or malfunction problems which might occur if the liquid were released to the environment.
The prior art has devised suitable equipment for achieving the withdrawal of liquid from a zero gravity storage tank containing both liquid and gas. U.S. Pat. No. 3,486,302 to Paynter discloses a system for recovering liquid from a storage vessel at zero or reduced gravity. A closed-volume woven wire mesh within the vessel is used to collect liquid by adhesion and cohesion forces, so that only liquid is passed from the mesh through an outlet in the vessel. A vapor outlet is provided in fluid communication with the volume between the tank and the interior closed-volume mesh, with gas passage through the outlet being responsive to pressure within the vessel. U.S. Pat. No. 3,933,488 to De Peri discloses a gas separator system which also employs a mesh screen in a vessel as a surface tension device to collect liquid and isolate the gas from the liquid. The mesh screen is collapsible as liquid is removed from the vessel at zero gravity. A charge opening is provided for inputting pressurized gas to the vessel. U.S. Pat. No. 4,272,257 to Ellion et al employs a liquid surface tension device of a different construction for use within a zero gravity liquid storage vessel. The device utilizes etched flow passageways which pass only liquid by surface tension effects, with gas being prevented from moving through the passageways. Tubular galleries are positioned within the tank so that liquid in preference to gas is delivered to the galleries. A vent line vents gas from the tank during the land-based liquid filling operation, and serves to supply pressurized gas to the tank.
While the prior art has devised systems for withdrawing liquid rather than a liquid/gas mixture from a storage tank at zero gravity, it has not provided equipment or techniques for effectively removing gas rather than a liquid/gas mixture from the storage tank at zero or near zero gravity. Similarly, prior art techniques have provided vent tubes for venting gas from a liquid storage tank at zero gravity, but such techniques are both complex and/or do not effectively prevent the discharge of liquid.
The disadvantages of the prior art are, however, overcome by the present invention, and simple yet reliable techniques are hereinafter disclosed for withdrawing substantially only gas from a liquid storage tank in a zero or near zero gravity environment. The techniques of the present invention allow the re-supplying of a non-cryogenic liquid to the tank in a zero gravity environment while venting only substantially gas from the tank to prevent an increase of pressure.